We do it every night, and over the course of our life we will spend approximately a third of our time doing it: sleep. But what is it? Doctors and scientists are really just beginning to understand all the important ways that sleep affects our health and well-being — and all the reasons why we do it.

According to Rafael Pelayo, MD, a clinical professor of psychiatry and behavioral sciences and a sleep specialist at the Stanford Sleep Medicine Center in Redwood City, California, “Sleep is a natural, restorative physiological process characterized by a perceptual disengagement [in which you tune out from whatever’s going on around you], and must be rapidly reversible.”

The National Institutes of Health define sleep as a complex biological process, during which you are unconscious but your brain and body are still active, that's necessary for learning new information, staying healthy, and feeling rested.

The bottom line is that we need sleep to function, Dr. Pelayo says. It’s a critical process that allows the body to function and stay healthy — and it’s especially important for the brain.

“The entire body takes advantage of sleep,” Pelayo explains. For example, research suggests the kidneys slow down production of urine, and digestion slows in the gut.

“But sleep is really how the brain gets reset for the next day. Sleep restores the brain.”

That means inadequate sleep or poor quality sleep will damage many systems of the body and over time can contribute to a greater risk of chronic disease and health problems. But the most immediate consequences of not sleeping that you’ll notice are those that affect your mind and thinking.

Common Questions & Answers

How much sleep do you need?

Adults ages 18 and older should get seven to nine hours of sleep each night, guidelines recommend. Adults over age 65 may need slightly less sleep, and children ages 17 and under need more sleep.

What’s the purpose of sleeping?

If you are not getting enough sleep, the chemical processes that flush out waste and replenish energy in the brain do not happen. This can make it tougher to concentrate, remember things, and have patience, and can make you more emotional than usual.

Does sleep affect your health in other ways?

Consistently sleeping poorly has been linked to worse heart health and increased risk of obesity, type 2 diabetes, depression, anxiety, and poor immune function. Lack of sleep also leads to a buildup of certain proteins that is linked to greater risk of developing Alzheimer’s disease and dementia.

What hormones affect sleep?

In addition to our body’s biological clock, cortisol and melatonin signal to the body when to wake and sleep.

How can you get better sleep?

There is no one-size-fits-all solution to sleeping better. Good sleep hygiene strategies, however, include sticking to a consistent sleep schedule, monitoring your caffeine intake, exercising regularly, and avoiding bright lights and screens close to bedtime.

Why Sleep Is So Important for Your Health

We intuitively know we need sleep. When you don’t get a good night’s sleep, you’ll likely feel drowsy, you won’t quite be able to think as clearly as usual, and you might be moody and irritable. That’s because one of the key functions of sleep is to restore the brain.

Learn More About What Happens to Your Body When You Don’t Sleep for Days

Why the Brain Needs Sleep

“Sleep is something the brain needs,” Pelayo explains. Our brains run on electricity, which means the chemical energy the brain uses to function has waste products (called metabolites) that need to get cleaned out. That’s what happens during sleep, Pelayo says. The brain flushes out that waste during sleep. The brain also experiences a spike in adenosine triphosphate (ATP), a molecule the brain uses for energy and that’s essential for communication between brain cells.

You likely won’t be measuring your daily ATP levels, but they do affect your ability to function in big ways. If you don’t get a good night’s sleep and those chemical processes don’t happen, the next day you’ll likely notice:

It’s tougher to concentrate.
It’s harder to remember things.
You’re moody and irritable.
Your judgment might be skewed.
You have less patience.
You’re more likely to make rash decisions or have a tough time making decisions.
You’re more emotional than usual.
Your hand-eye coordination is a little bit off.

There’s also emerging evidence that over time, regularly not getting enough sleep leads to the buildup of certain proteins in the brain that are linked to Alzheimer’s disease and other neurological problems.

Why the Body Needs Sleep

Of course, it’s not just our minds that need sleep. Other systems of the body don’t work quite right when they’re too tired, either. Immediately after a poor night’s sleep you might notice you’re hungrier and tend to crave and eat more,

and people are also at higher risk of catching a cold or flu.

Researchers think that’s because sleep deprivation has been shown to mess with how the immune system functions.

Over time, chronic poor sleep has been linked to worse heart health. So much evidence points to this that the American Heart Association updated its checklist of modifiable factors linked to cardiovascular health in June 2022 to include sleep. The list also includes diet, exercise, tobacco use, weight, cholesterol, blood sugar levels, and blood pressure, and is published in the journal Circulation.

Sleeping poorly over time has also been shown to increase the risk of:

Obesity
Type 2 diabetes
Heart disease and hypertension
Depression, anxiety, and other mood disorders
Poor immune function
Earlier death

Learn More About Why Not Getting Enough Sleep Is Bad for Your Health

Circadian Rhythm, Sleep Stages, and Sleep Cycles: Everything You Need to Know About What Happens When You Sleep

You may not remember everything that happens each night when you’re asleep, but if you’re doing it right, there’s a lot going on in your brain and your body, Pelayo says. “There are differences between sleep and awake for every single body system, but nothing as dramatic as the changes of consciousness during sleep,” he says.

The Different Stages of Sleep

During sleep the brain cycles, repeatedly, through different stages.

Stage 1: Non-REM (rapid eye movement) sleep The first stage is when you’re falling asleep — stage 1 non-REM. Your heartbeat, breathing, and eye movement start to slow down, and your muscles relax. Your brain waves also slow down, and it’s still very easy to wake up during this preliminary stage of sleep.

Stage 2: Non-REM sleep In the second stage, your heart rate drops and your body temperature falls even more. Eye movement stops completely and your brain slows way down, except for brief bursts of activity.

Stage 3: Non-REM sleep Next comes deep sleep. This stage is heavy and restorative. Your heartbeat and breathing slow down the most during this type of sleep, and now is the time when it's hardest to awake.

REM sleep Finally comes REM sleep, when your eyes begin to dart quickly back and forth from side to side (even though your eyelids are still closed). Brain activity speeds way up, closer to the amount of activity that happens when you’re awake. This is the stage of sleep when most of your dreaming happens. Your breathing speeds up and becomes irregular during REM sleep. Heart rate and blood pressure start to climb back to waking levels, but the muscles of your arms and legs become temporarily paralyzed. Sleep experts suspect this paralysis is a mechanism our bodies developed to protect us from injury or other harm that might otherwise ensue if we were to “act out” our dreams.

Each cycle of sleep (a set of all the stages) usually takes about 90 minutes. And most people tend to spend more time during each cycle in deeper sleep earlier in the night — and more time in REM sleep later on. Each stage of sleep is important, and both deep sleep and REM sleep play critical roles in the learning and memory consolidation processes that happen during sleep.

Learn More About the Sleep Cycles and the Stages of Sleep

What Drives Sleep

Two internal systems control when we sleep and when we’re awake. First, there’s the sleep-wake homeostatic drive. The longer we’re awake, the more our bodies crave sleep — and the longer we’re asleep, the more the body wants to wake up.

The homeostatic sleep drive affects how deeply we sleep, too. For instance, if you stay awake for 24 or 36 hours instead of the typical amount of time you spend awake during a day, such as 16 or 17 hours, sleep-wake homeostasis is the mechanism that drives you to sleep longer and deeper once you do sleep.

Then there’s our circadian rhythm, our body’s biological clock, which syncs our body functions with environmental cues. These internal clocks are what drive us to feel sleepy at night and more awake in the morning (even, for instance, if you slept poorly the previous night, or pulled an all-nighter). They’re regulated by hormones, such as the stress hormone cortisol and the sleep hormone melatonin, which get secreted by the brain to send these wake and sleep signals to the body.

“They’re two complementary systems in the brain,” Pelayo says. And when there’s a discrepancy between the homeostatic drive to sleep and the signal to sleep that comes from the circadian system, problems like jet lag and other disordered sleep occur.

“This is why people who wake up at different times every day may feel tired a lot,” Pelayo says. “The brain doesn’t know how to predict when they should be awake. It’s like being constantly jet-lagged.”

The more sleep researchers learn about these two systems that control sleep, the more it is clear why not only sufficient hours of sleep, but also good sleep habits (such as going to sleep and waking up at the same time each day) are important.

Learn More About Your Circadian Rhythm and How It Affects Sleep

How Much Sleep You Actually Need

How much sleep you need each night varies somewhat depending on your age (younger people typically need more sleep than older adults) and our genes (some people are naturally shorter sleepers than others). But typically the sleep target for adults is between seven and nine hours each night, according to guidelines from the National Sleep Foundation.

That recommendation, along with additional recommended sleep times for younger children, adolescents, and older adults, is based on the amount of sleep associated with the best health outcomes in a number of areas, including things like mood, learning, accidents, hypertension, heart disease, diabetes, and pain.

But Pelayo says not to get too concerned about banking a specific number of hours of sleep each night. “The issue is waking up refreshed,” he says. “You should never wake up tired. If you do wake up feeling tired, something is wrong.”

Waking up sleepy may indicate that the quality of your sleep is poor. Maybe you’re spending too much time in light sleep and not getting enough restorative deep sleep, for example, Pelayo says. If that’s the case, you should ask your doctor about getting checked for a sleep disorder, or see a sleep medicine specialist.

Learn More About How Much Sleep You Need Each Night

Common Sleep Disorders

Everyone should be able to get a good night’s sleep on a regular basis, Pelayo says. And if you’re not (and it’s not because you lack the opportunity to sleep), it’s important to be aware of the several sleep disorders that might be interfering with your rest.

Below are some of the more common sleep disorders and some signs that you may have one.

Insomnia Insomnia is characterized by difficulty falling asleep or staying asleep. Cases can be short term, such as those due to a stressful event, like a job change or jet lag; or long term, meaning the sleep trouble lasts for three months or longer, which is known as chronic insomnia.

Obstructive sleep apnea (OSA) Obstructive sleep apnea, or “sleep apnea” for short, is a disorder in which a person's airway becomes partially or completely blocked during sleep, causing the person to repeatedly wake up and preventing the deep, restorative sleep they need. People who are obese, have a small jaw or a large overbite, or use alcohol before bed are all at a higher risk for sleep apnea.

If you snore or wake up still feeling tired, particularly after a full night asleep, you may have sleep apnea and should get checked out by your doctor. Left untreated, sleep apnea can cause big problems, including high blood pressure, heart disease, memory problems, and higher accident risk.

Narcolepsy Narcolepsy is a disorder of the central nervous system in which the brain cannot properly regulate cycles of sleep and waking.

People with the disorder can experience the sudden, sometimes uncontrollable, need to fall asleep throughout the day, as well as trouble staying asleep at night.

Restless legs syndrome (RLS) RLS is a disorder that causes uncomfortable sensations in the legs and an irresistible urge to move them.

Symptoms are most likely to occur when you’re sitting, resting, inactive for a while, or sleeping. The condition is categorized as a neurological sensory disorder because the symptoms come from the brain — though it is also classified as a sleep disorder. It can cause exhaustion and daytime sleepiness that affects mood, concentration, learning, and relationships.

Parasomnias A parasomnia is term used to refer to a number of disorders associated with abnormal behaviors that happen during sleep. Parasomnias include sleepwalking, sleep-related eating disorder, sleep terrors, bed-wetting, sexsomnia, and others. In some cases, improved sleep habits can alleviate parasomnias, and in other cases treatment by a sleep medicine doctor may be needed. You should definitely seek treatment if abnormal behavior associated with sleep is causing harm to yourself or others, or if the behavior is frequent or escalating.

None of these problems should be left unaddressed, Pelayo says. If you suspect you may have one of these conditions, it’s important to get checked out and treated.

Learn More About These and Other Sleep Disorders

How to Sleep Better Tonight

There’s no silver bullet formula for getting a good night’s sleep, but there are several steps you can take that have been associated with better sleep overall if you’re struggling to clock the recommended number of hours you know you need — or if you wake up less perky than you'd like to be.

It’s important to check with your doctor or a sleep medicine specialist if you think you have a more serious problem, or if another medical condition is interfering with your sleep.

But trying these fixes is a fine place to start.

Stick to a consistent sleep-wake schedule. Aim to go to bed at the same time each night and wake up at the same time in the morning, including on the weekends — and try not to vary it by more than an hour or so. The times that you regularly go to bed and wake up are the signals you give your body’s natural clock, and when they’re consistent, that clock helps you wake up and fall asleep. If those signals are out of whack, your body clock gets thrown off and you experience the same drowsiness associated with jet lag. You also may struggle to fall asleep at night or wake up when your alarm rings.

Watch caffeine intake. Be especially careful with this later in the afternoon. Pelayo suggests avoiding caffeine within six hours of when you want to sleep.

Exercise regularly. Research shows that regular exercise (at least 150 minutes of activity per week) is associated with better sleep, though it’s worth noting you should try to avoid intense exercise too close to bedtime, as it may make it tougher for some people to fall asleep.

That's because a workout sends signals to the body, such as increased heart rate and body temperature, that tend to wake you up.

Avoid bright lights and bright screens right before bed. Blue light — the kind that comes from fluorescent bulbs, LEDs, and computer and cellphone screens — has been shown to send the same signals to the brain as sunlight, and blocks production of the hormone melatonin, which tells the brain to go to sleep.

If you can’t sleep, don’t linger in bed. This means at night if you’re having trouble falling asleep for 20 minutes or longer, get out of bed and do something to make you tired, such as reading or some gentle stretching. Staying in bed makes your body associate in-bed time as awake time, and it will actually be harder to fall asleep.

Don’t linger in bed in the morning either, and don’t hit snooze. It can be tempting to wake up slowly, but that drowsy sleep (after you’ve initially woken up) is fragmented, light sleep. If you did get a poor night’s sleep, your best remedy is getting up, going about your day, and hitting your pillow at bedtime that evening, at which point your sleep drive will be strong and you’re more likely to actually reap the benefit of the deep restorative sleep you need.

Learn More About How to Fix Your Sleep Schedule

Resources We Love

Whether you’re trying to improve your sleep or need to learn more about various sleep disorders, here are some dreamy sleep resources.

American Sleep Apnea Association

If you’ve just been diagnosed with sleep apnea, connect with this association. It includes resources, such as a CPAP Mentor program, matching you with an experienced sleep apnea patient, a CAP Mask program, which provides equipment to those who can’t afford it, and a library of webinars and podcasts about living with sleep apnea.

Project Sleep

This nonprofit is all about raising awareness of sleep health and sleep disorders. Learn more about the benefits of sleep, common sleep disorders, and how to find a sleep center near you. Project Sleep runs a slate of programs, including the Jack and Julie Narcolepsy Scholarship supporting students with narcolepsy and hypersomnia, and the Rising Voices of Narcolepsy program, helping the next generation of narcolepsy patient-advocates spread awareness about the sleep condition.

Narcolepsy Network

Founded in 1986, this longtime national nonprofit provides emotional support and resources to patients, family members, and friends. It includes the latest clinical research on narcolepsy, support groups for various age groups, and details on sleep centers and narcolepsy specialists across the country.

Restless Legs Syndrome Foundation

Established in 1992, this nonprofit is home to science-based education and patient services to help people living with the syndrome. It claims to be the only organization with a dedicated grant program to advance research for new treatments and a cure for restless legs syndrome.

SLEEP

SLEEP is the international journal of sleep and circadian science. If you want to read the latest peer-reviewed research and commentaries, or look up existing research on specific sleep disorders, it has a vast collection of papers to browse through. Topics include circadian disorders, insomnia, sleep and metabolism, and the neuroscience of sleep.

Sleep Education

Established by the American Academy of Sleep Medicine, this website has a bedtime calculator, sleep diaries, and explainers on dozens of sleep disorders, including sleep-wake disorders, hypersomnias, parasomnias, breathing disorders, and movement disorders.

The Sleep Doctor

Created by the clinical psychologist and American Academy of Sleep Medicine Fellow Michael Breus, PhD, this website calls itself “your ultimate sleep resource center.” It includes tips on how to sleep better, doctor-recommended products to help with your slumber, and sleep quizzes developed by Dr. Breus to help you figure out your sleep chronotype.

The Sleep Lady

If you’re sleep training your little ones, make sure to bookmark this website on your browser. Kim West, LCSW, offers parents with newborns to 6-year-olds sleep tips, including how to handle nightmares and napping and how to develop a sleep schedule for every age and life stage. She's also written a book aptly titled Good Night, Sleep Tight.

Editorial Sources and Fact-Checking

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Sleep and Mood. Harvard Medical School. December 15, 2008.
Judgment and Safety. Harvard Medical School. December 16, 2008.
Ju ES Ooms, SJ, et al. Slow Wave Sleep Disruption Increases Cerebrospinal Fluid Amyloid Levels. Brain. August 2017.
Spiegel K, Tasali E, et al. Brief Communication: Sleep Curtailment in Healthy Young Men Is Associated with Decreased Leptin Levels, Elevated Ghrelin Levels, and Increased Hunger and Appetite. Annals of Internal Medicine. December 2004.
Prather AA, Leung CW. Association of Insufficient Sleep With Respiratory Infection Among Adults in the United States. JAMA Internal Medicine. June 2016.
Watson NF, Buchwald D, Delrow JJ, et al. Transcriptional Signatures of Sleep Duration Discordance in Monozygotic Twins. Sleep. January 2017.
Sleep and Chronic Disease. Centers for Disease Control and Prevention. August 2018.
The Stages of Sleep. Sleep Foundation. December 2021.
Brain Basics: Understanding Sleep. National Institute of Neurological Disorders and Stroke. August 2019.
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Sleep, Learning, and Memory. Harvard Medical School. December 2007.
Hirshkowitz M, Whiton K, et al. National Sleep Foundation’s Sleep Time Duration Recommendations: Methodology and Results Summary. Sleep Health. January 14, 2015.
Sleep Disorders. Sleep Foundation. December 1, 2020.
Insomnia. Sleep Foundation. January 31, 2022.
Narcolepsy Fact Sheet. National Institute of Neurological Disorders and Stroke. November 15, 2021.
Restless Legs Syndrome Fact Sheet. National Institute of Neurological Disorders and Stroke. November 15, 2021.
Parasomnias. Sleep Foundation. January 20, 2022.
Loprinzi PD, Cardinal BJ. Association Between Objectively-Measured Physical Activity and Sleep, NHANES 2005–2006. Mental Health and Physical Activity. December 2011.
When to Exercise for a Better Night’s Sleep. Sleep.org. March 5, 2021.
Blue Light Has a Dark Side. Harvard Medical School. July 7, 2020.
Lloyd Jones DM, Allen NB, Anderson CAM, et al. Life’s Essential 8: Updating and Enhancing the American Heart Association’s Construct of Cardiovascular Health: A Presidential Advisory From the American Heart Association. Circulation. June 29, 2022.

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Sleep 101: The Ultimate Guide to a Better Night’s Sleep